1. Field of the Invention
This invention relates to pressure measurement and more particularly to a high resolution and large dynamic range pressure sensor that uses oscillator resonance and the associated Q-factor in order to determine such pressure.
2. Description of the Related Art
With increased technical knowledge with respect to material science, machine fabrication, and operating modes, pressure sensors have become increasingly refined. In particular, micromachined quartz or silicon pressure sensors have been the subject of research and industrial use for several years. Such micromachined pressure sensors take generally two forms: diaphragm-based pressure sensor and vibrating beams. Such pressure sensors are often made of either quartz or silicon and a summary of such materials and their characteristics are given in the article M. Dufour et al., A Comparison Between Micromachined Pressure Sensors Using Quartz or Silicon Vibrating Beams, Sensors and Actuators A. 34, 1992, 201-209. Other articles and descriptions of the current state of the art describe various aspects and considerations with respect to such micromachined pressure sensors.
As a basis for using such micromachined pressure sensors, changes occur in the operating characteristics of such pressure sensors when subject to varying types and/or amounts of pressure. For some designs, piezoelectric characteristics serve as means by which pressure may be detected. For other designs, change in resonant frequencies due to the presence of pressure serves as the means by which pressure is detected. Such pressure sensors are generally inexpensive, highly reliable, very accurate, and easy to adapt to a variety of tasks in a variety of environments. By expanding the available variety and types of micropressure sensors, greater pressure sensing and associated economic and industrial advantage is achieved.
Any resonator, such as mass on a string, has a natural frequency at which it will preferably oscillate. The degree to which energy can be stored in a resonator in relation to the energy dissipated per half-cycle is related to the quality factor, or Q-factor, of the resonator. With the mass on the string example, Q is equal to the resonant frequency divided by a factor .gamma.. .gamma. is equal to the damping rate which is equal to .eta. divided by the mass. .eta. represents a viscous damping coefficient representing frictional dissipation. This example with respect to the simple case of the mass on a string can be extended to almost any resonator. The equation is given below EQU Q=.omega..sub.0 /.gamma.=.omega..sub.0 m/.eta.
By analogy, it can be seen that when pressure upon a micromachine resonator changes, the factor .eta. may change as well. Additionally, the natural resonant frequency of the resonator may change. For any particular resonator, these factors are generally known and generally do not change with respect to different, but identically constructed, resonators. Consequently, by evaluating the Q-factor of a resonator, the pressure upon that resonator may be evaluated. For micromachined pressure sensors, such pressure evaluation can be extremely accurate due to the high Q of the resonator coupled with the very low power operating requirements. Prior to the present invention, use of a resonator's Q-factor in a micropressure sensor has not been achieved and/or exploited.